1. Field of the Invention
The field of invention relates to zeolites. More specifically, the field relates to partially collapsed zeolites for the purification of natural gas.
2. Description of the Related Art
Almost one quarter of the total worldwide production of energy is met through natural gas production. Regulations for the transportation of natural gas that occurs mainly through pipelines vary by country. Usually there are specific restrictions to the amounts of inerts (mainly in the form of nitrogen) and carbon dioxide. Nitrogen is found in wellhead gas in range of about 0.5 to about 5 mole percent (mole %); however, from some sources can be as high as 30 mole %. Sub-quality natural gas has a nitrogen concentration greater than 4 mole %. Carbon dioxide is usually found in a range of about 0.2 to about 1 mole % of the wellhead gas. Sub-quality natural gas has a carbon dioxide concentration greater than 2 mole %. Unfortunately, carbon dioxide is an acid gas and with water it forms carbonic acid, which reacts with carbon steel and other metals susceptible to acidification and corrodes them, especially in areas along the pipeline where pools of aqueous liquids form. Both gases have no heating value, so both reduce the thermal quality of the wellhead gas.
Carbon dioxide is normally removed by amine scrubbing within gas-liquid contactors at 323-333° K. The saturated alkanolamine is regenerated at 383-403° K. and the pure carbon dioxide released. This process is energy intensive and involves the handling of a corrosive and toxic solvent. The removal of nitrogen from methane is very difficult. The only commercial process for separating nitrogen from methane is cryogenic distillation, where a turboexpander reduces the temperature of the gas to about 220° K. The nitrogen-poor product stream must be recompressed to effectively transport it through pipelines. Both turboexpansion and recompression are energy-intensive.
Adsorption processes using zeolites show capability to perform certain CH4—CO2 and CH4—N2 separations. Molecular Gate® (Engelhard Corp.; Iselin, N.J.) uses titanosilicate-based zeolites (ETS and CTS configurations) doped with transition metals, which allows for the micropores of the zeolite to be adjusted based upon activation temperature. Other adsorbents include carbon molecular sieves for CH4—N2 separations. A pressure swing adsorption (PSA) system using metal-exchanged clinoptilolites have also shown some promise for CH4—N2 separation. Testing using CMS 3A (carbon molecular sieve 3A) for performing CH4—CO2 separations has also occurred.
As a selective adsorbent for N2 and CO2, zeolite-based materials are attractive candidates. Zeolite 13X, which is an aluminosilicate zeolite, has been shown to remove some carbon dioxide from flue gases at low temperatures. Zeolites are thermochemically stable, available in the market and the surface can be controlled through post-modifications to the zeolite such as ion-exchange. Most importantly, zeolites have well-defined microporous structures that have a mean diameter range of about 0.3 nanometers (nm) to about 1.5 nm. For this size of mean diameter range, the zeolite can provide a molecular sieve effect in separating certain natural gas constituents from others.
Despite the advantages of zeolites, the separation of N2 and CO2 from CH4 is challenging. The extremely small difference between the kinetic diameters of the compounds (CO2: 0.33 nm; N2: 0.36 nm; CH4: 0.38 nm) requires precision in forming the zeolite apertures. It should be noted that the pore diameter of zeolites (and also other materials as well) is difficult to control in ultra-small pore range (mean diameters that are less than 0.38 nm). The attraction of titanosilicate-type ETS-4 for small molecular separations is attributable to its pore size tuning. Titanosilicate materials have two significant problems. First, they have lower thermal stability, so it is more difficult to use them in processes using thermal cycling to promote adsorption and desorption. Second is lack of availability. Aluminosilicate-based zeolites are more common and much less expensive than titanosilicate-based zeolites.